Load-supporting structure, particularly for marine wells



March 7,1967 ALUBINSK. I 3,307,624

LOAD-SUPPORTING STRUCTURE, PARTICULARLY FOR MARINE WELLS Filed May 22, 1963 "r Sheets-Sheet 1 20 I W 230 j 2O 2 H J Hcw El 7 2| 2| f f K. & \Y {x v A\ ARTHUR LUBINSKI INVENTOR ATTORNEY.

March 7, 1967 A. LUBINSKI 3,307,624

LOAD-SUPPORTING STRUCTURE, PARTICULARLY FOR MARINE WELLS Filed May 22, 196-3 '7 Sheets-Sheet 2 ARTHUR LUBINSK! INVENTOR ATTORNEY.

FIG-7 March 7, 1967 A. LUBINSKI 3,307,624

LOAD-SUPPORTING STRUCTURE, PARTICULARLY FOR MARINE WELLS Filed May 22, 1963 7 Sheets-Sheet 5 ARTHUR LUBINSKI INVENTOR.

A TTORNE Y.

March 7,1967 A.-LU B|NSK| 3,307,624

LoAmsuPPoaTiue STRUCTURE, PARTICULARLY F R MARINE WELLS Filed May 22, 1963 "r Sheets-Sheet 4 ARTHUR LUBINSKI INVENTOR.

. ATTORNEY.

March 7, 1967 A. LUBINSKI 3,307,624

LOAD-SUPPORTING STRUCTURE, PARTICULARLY FOR MARINE WELLS Filed May 22, 1963 7 Sheets-Sheet 5 HG. -l2

ARTHUR LUBINSKI INVENTOR.

ATTORNEY.

A. LUBINSKI March 7, 1967 LOAD-SUPPORTING STRUCTURE, PARTICULARLY FOR MARINE WELLS Filed May 22, 1963 7 Sheets-Sheet 6 COM COM

mmI02 mmhmi o dO L M0139 M3210 MTGE ARTHUR LUBINSKI INVENTORA ATTORNEY.

LOAD-SUPPORTING STRUCTURE, PARTICULARLY FOR MARINE WELLS Filed May 22, 1965 7 A. LUBINSKI 7 SheetsSheet March 7, 1967 ll, W mToE Q QE m B U L Du U m flzotom 29mm EEQQ zwzmn E M 87501 6 85 05 E zowwzo 6 Emma $102.- EESZE 232;; com 00 com com 0Q o y om. ow o 0 1 u m m H W M v mz3 M W M V 7 -52 o3 m n d M I. w H i oom l- 0 d 0 00m A O W S S O 09 N m Ill w q o wofimpw $22,

INVENTOR.

ATTORNEY.

United States Patent 3,307,624 LOAD-SUPPORTING STRUCTURE, PARTICU- LARLY FOR MARINE WELLS Arthur Luhinski, Tulsa, Okla, assignor to Pan American Petroleum Corporation, Tulsa, Okla., a corporation of Delaware Filed May 22, 1963, Ser. No. 282,462 14 Claims. (Cl. 166-.5)

This invention relates to load-supporting structures and is directed particularly to a structure adapted for drilling and producing operations in connection with marine oil and gas wells.

Off-shore wells which are completed for the production of oil or gas, or both, may generally be considered to fall in one of three categories, as regards the location of the wellhead or Christmas tree which carries the control valves and which supports the Well casing string or strings. The wellhead may be located above the water surface, below the water surface but within diving range, or beyond diving range on the marine floor. With present diving techniques and equipment, work at a depth of 100 feet is routine. At 200 feet depth, working time is short. A descent to 300 feet of depth would only be tried in case of serious emergency. Thus, diving range is now considered to be from zero depth to depths of between 100 and 200 feet. For marine floor wellheads beyond diving range, complex and expensive remote control apparatus, which is not always completely reliable, must be provided for both installation and operation of the well equipment.

When the wellhead is above the water surface, or below it at such a depth as to be accessible to a diver but still a substantial distance above the marine floor, it has heretofore been considered necessary to use a caisson surrounding the well casing. The caisson is essentially a large, hollow column driven into the marine floor for a sumcient depth to provide stable support for the weight of the column itself, for the wellhead equipment, and for the very large weight of the casing strings which it is required to support. Heretofore, the fact that the very large weight of the casing strings is applied at its top has been considered to require that the caisson be extremely heavy and bulky to support the heavy loads and withstand the very high resulting compression without buckling. In view of the experience heretofore gained in drilling with caissons in water depths generally accessible to divers, it has been generally considered that caissons will not be practical for much greater depths because of their large size which makes them both diificult to handle and high in cost.

In view of the foregoing, it is a primary object of my invention to provide a novel structure of the caisson-type which substantially avoids the relative disadvantages of cost, size, and weight noted above, particularly for drilling and production in water depths some hundreds of feet greater than diving range. A more specific object is to provide a caisson-type of supporting structure for use in marine drilling and producing operations where the wellhead is either above or below the water surface, but especially where it is below the surface in a range of water depths, both within and somewhat greater than diving range, which structure saves much of the weight, expense, bulk, and difiiculty of manufacture and installation of the heavy caissons heretofore utilized. A still further object is to provide a caisson construction which utilizes the structural materials with increased efliciency, by taking advantage of existing forces to provide stability instead of merely increasing the bulk or the thickness of the caisson walls. A still further object is to provide a caisson-type supporting structure of reduced size over those heretofore employed for supporting given loads,

so as to be less subject to the external forces of wind and waves and, therefore, to minimize resulting problems. Still another object is to provide a caisson-type structure substantially free of limitation as to the depth of water in which it can be economically utilized for drilling and production operations, or as to the depth of wells which may be drilled utilizing it, except insofar as the weight and bulk of the caisson itself required for it to be substantially self-supporting, become too large for handling. Other and further objects, uses, and advantages of the invention will become apparent as the description proceeds.

Briefly stated, my invention is based on the discovery that, by insuring that the various members of the structure and of the load it supports remain in a proper geometric relationship, the very large force of the tension in an inner member suspended within an outer caisson member can be converted into very effective stabilizing forces substantially decreasing bending of the caisson member due to the high compression to which it is subjected by the very weight which is responsible for the tension. That is, the caisson structure of my invention is in a large degree a self-stabilizing system, in that the greater the load carried by an inner member in tension which tends to cause bending or buckling of the caisson, the greater is the force available to oppose the bending.

In its simplest terms, this means essentially that the inner tension member (casing) must be centralized within the outer tubular caisson member to a sufiicient extent, or the clearance between the tension member and the compression caisson member must otherwise be maintained small enough, so that the bending of the caisson due to the compression imparted to it by the Weight of the casing encounters an opposing force from the tension in the casing.

The meaning of the foregoing and the manner in which it is employed in the design and construction of various types of marine drilling and producing structures will be better understood by reference to the accompanying drawings forming a part of this application. In these drawings,

, FIGURES 1 through 7 are diagrammatic cross sections of various elementary column structures showing the principles upon which the invention is based;

FIGURE 8 is a graph showing the relationship between column height and load-supporting capacity of a representative caisson material under various conditions;

FIGURES 9 through 12 are schematic cross-section views of various embodiments of a caisson structure showing its adaptation to various marine drilling and producing operations;

FIGURE 13 is a cross-section view, similar to FIG- URES 9-12, of a preferred caisson structure utilizing the invention; and

FIGURES 14 through 16 are g aphs showing certain properties of the caisson of FIGURE 13.

In order to understand the invention, it is important first to understand the concept of critical column length or height. Consider FIGURE 1. A vertical column 20 is rigidly attached at its base to a horizontal surface 21, which is typically the surface of the solid earth. In practice, the column 20 generally extends some distance into the earth as indicated by the dotted outline 22, in order to find load-bearing support by contact With the bedrock or frictional engagement with the surrounding earth medium. For the purposes of this simplified explanation, however, the portion 22 can be neglected simply by assuming that the base of column 20 is rigidly fastened to surface 21.

While the column 20 is illustrated as a single hollow tubular member having a finite wall thickness, it may consist of two or more concentric tubular members with cross bracing between them to maintain concentricity;

ment continues until the column buckles and falls.

or it can assume a variety of still ditferent forms. That is, other than being tubular, the column might be in the form of a solid member of circular or other cross-section, or it might consist of a plurality of such members spaced apart and interconnected by bracing and the like. Regardless of its particular form, the column 20 has a certain effective cross section and weight per unit length. It also is formed of a material having known mechanical properties of strength and rigidity. Keeping these factors in mind, as the height or length of column 20 increases, its weight similarly increases. Obviously, for a very great height critical conditions of stability will be reached, and the column will buckle and collapse under its own weight. For small heights the column is more than sufiiciently strong to support itself. Accordingly,

. for any given material and cross-sectional configuration of the column, there is a definite height where the column just loses its ability to be self-supporting. This is termed the critical length or critical height and is designated in FIGURE 1 as H If the column is required to support an additional load, such as the weight W in FIGURE 2, this load will reduce the height of the column that is just able to support both its own weight and the weight W. This length or height of the column 20 may be termed a critical loading height, analogous to H and designated H H is thus the critical column height where the load W is just stably supported, with the meaning that if the weight W is increased by even the smallest amount, or the height of the column 20 is increased by the smallest amount, the column 20 buckles and falls.

Now consider FIGURE 3. With the column 20 at its critical height H where it is just able to support its own weight, the additional weight W is now applied to the top of the column by hanging it on a tension member 23 attached at the center of the top of the column and extending into a cavity 24 below the bottom of column 20 through an opening in the earths surface 21 assumed to be made without disturbing the attachment of the base of column 20 to this surface. Based upon the concept of critical height as stated above, this places column 20 in an unstable condition where it must buckle.

What actually happens is illustrated in a general way by FIGURE 4. The top of column 20 deflects laterally as the column bends under the applied total load, which is greater than its ability to support, and the tension member 23 and weight W similarly move in the direction of the deflection of the top end of column 20. This move- The force responsible for this collapse can be understood by noting that, with weight W centered in the cavity 24, it is at a height I1 above the floor of the cavity. In FIGURE 4, however, as weight W swings to the side of the cavity, it drops to a lesser height h This means that the potential energy of the weight W at its position in FIGURE 3 is greater than at its position in FIGURE 4; and the difference in the potential energy is converted into the strain energy of bending of the column 20 at its point of attachment to base 21 during the process of buckling.

Now suppose that by some means the tension member 23 is constrained from being decentralized in column 20 as in FIGURE 4, but remains centered within the column as it bends. Then, if a force F acting laterally at the top of the column in FIGURE moves it to the position shown in FIGURE 4, while the tension member 23 remains centralized in the column 20 as in FIGURE 5, the weight W in the cavity 24 does not move and, therefore, no bending strain contributing to the buckling of column 20 is produced by it. In other words, this means that if the tension member 23 is maintained in the center of the column, the weight W is without efiect on the column stability, and the column 20 is able to support both its own weight and the weight W, even though its height H is such that even the smallest fraction of weight W applied to the top of the column rather than through the tension member 23, would cause it to buckle.

In a practical sense, this means that a load-bearing column 20 carrying a weight W at its top as shown in FIGURE 6, as long as it does not exceed the length or height H can additionally support through a tension member 23 attached to the midpoint of the top of the column, a weight M attached to the tension member 23, provided there are a sufficient number of stabilizing members 25 between the tension member 23 and the compression colu-mn member 20 to keep the tension member 23 centralized in the center of the column. The weight M can even be much greater than the weight of the column 20 and external weight W combined. Qualitatively, the functioning of the centralizers 25 can be visualized as simply utilizing the tension of the member 23 which tends to keep it straight, to provide a lateral force that efiectively opposes the tendency of the column to bend or buckle, just as soon as bending establishes a force-transmitting contact between the member 23, the centralizer 25, and the inner wall of column 20.

FIGURE 7 shows diagrammatically an equivalent and generally preferred form of supporting structure utilizing the invention in which the tension member 23 has an outside diameter only slightly smaller than the inside diameter of column 20, by an amount required for minimum operating clearance. A very small bending of outer column 20 under load then brings it and member 23 into contact over most of their lengths; and the weight M remains substantially motionless, thus creating no further bending stress.

FIGURE 8 shows graphs of the column stability of a typical material from which a column might be made for use in some of the applications to be described below. Specifically, the graphs of FIGURE 8 apply to a column made out of a standard 20-inch outside diameter, 94 pounds per foot, API steel casing. These dimensions, material, and weight establish a certain critical height H The lower curve of FIGURE 8 labeled No Inside String, however, shows how this critical height must be reduced in order to support various external loads without buckling. In other words, this curve corresponds to the conditions illustrated in FIGURE 2, with abscissas of the curve corresponding to various values of the weight W. It may be observed that it is largely immaterial to the column stability whether the weight W is imposed at the top end of the column directly as in FIGURE 2, or as tension through an uncentralized tension member 23 attached in the center of the column cap as in FIGURE 3, with the height of the column in FIGURE 3 reduced to its value H as in FIGURE 2. For example, if the column of 20-inch casing is required as a caisson to support a load W of 200,000 pounds at its top as in FIGURE 2, FIGURE 8 shows that its load-bearing height H can be only about 30 percent of its critical height H, when unloaded. If, however, this same column is provided with a centralizer only at its bottom, and the load is supported from the center of the top of the column by the tension member 23, the presence of the bottom centralizer allows supporting almost an additional 500,000 pounds of load, \for a total of nearly 700,000 pounds of stably supported weight. Alternatively, with a bottom-end centralizer and with the welght supported from the center of the top, the 200,000- pound load can be supported as tension on the inner member by a column which is almost 60 percent of the critical unloaded column height. This is nearly twice the height of the stable column capable of supporting this lo ad without centralization.

If in addition, a centralizer is placed also at the center of the column, so that the tension member is centered at the top, center, and bottom of the column, the reduction in height of the stable column otherwise capable of supporting only its own weight is only about 13 percent, for the purpose of supporting an additional load of 200,000 pounds through the tension member 23. More importantly, to support very large loads much lighter-weight and therefore much less expensive and more easily handled columns can be employed than have been heretofore used, when such loads can be applied through a tension member passing through and maintained in the center of the column.

FIGURE 9 shows how the invention may be applied to the subsurface completion of an oil or gas well in a marine location. Thus, the caisson 20 is driven to a loadsupporting depth below the marine bottom 21. The top of caisson 20 carrying the slip bowl 29 and slips 30 is located at a suitable depth below the surface of the water 32 so as to be accessible for work by divers sent down from the water surface, but sufficiently below the surface to offer no hazard to navigation. That is, the wellhead control mechanism 33 can be reached from the water surface for installation, adjustment, maintenance and the like. In this case, the caisson member 20 need be only of sufficient strength with respect to bending to support its own weight plus that of the slip bowl and slips 29 and 30 and the wellhead equipment 33. For support of the very large weight carried by the casing 23, it is necessary only to provide a sufiicient number of centralizing devices 25 between'the caisson 20 and the casing 23, within the depth range from the casing bowl 29 to the mud line 21. Obviously, the centralizers 25 are not necessary if the outer diameter of casing 23 and inner diameter of caisson 20 are very nearly the same, so that the clearance between them is small. By small clearance is meant clearance such that only a small amount of bending (much less than it is capable of withstanding) of caisson 20 brings it and casing 23 into contact over most of their length, as in FIGURE 7. With this arrangement of elements, the conditions in effect are those illustrated by FIGURES 6 and 7, so that substantially any desired load may be supported by the casing 23 below the level of mud line 21, without the structure becoming unstable.

The showing of wellhead 33 within diving range below the water surface in FIGURE 9 should not be interpreted to mean that the advantages of the invention are limited to subsurface well completions. On the contrary, for well completions where the wellhead is at. a safe height above the water surface, utilization of the invention can make an important saving. That is, the caisson structure of FIGURE 9 might be extended above the water surface. Centralizing the casing strings, either by small clearance or annular spacers in accordance with the invention, allows bending stresses due to the casing weight to be neglected in the caisson design.

In FIGURE is an embodiment of a supporting structure illustrating the use of auxiliary stabilizing means in conjunction with the invention. Thus, in Water depths where the length of the caisson 20 is too great to be selfsupporting, so that it would buckle or even prior thereto would bend excessively due to its own weight, this tendency may be counteracted in either or both of two ways. The column may be stabilized by one or more sets of guy cables 27 extending from appropriate points along the length of the column to anchors or anchoring piles 28 set at some distance away in the marine bottom 21, in three or more equiangular directions from the base of the column. Alternatively or in addition, a substantial portion of the weight of the column may be supported by a buoyancy chamber 34 concentrically surrounding the column 20 near its upper end. By either or both of these means, column 20 may be stabilized substantially without regard to its total length, while by providing a small clearance or a s-uificient number of centralizers 25 between the column 20 and suspended tension member 23, any additional weight which appears as tension in the member 23 is substantially without effect on the bending and stability of the column structure.

In FIGURE 11 is shown one manner of utilizing the invention in the course of a marine drilling operation. The caisson 20 for a marine well location is ordinarily set in place as one of the first steps in the drilling operation. Next is installed the surface casing 23, here shown as of a diameter to have only small clearance with the inside of caisson 20. A floating drilling vessel 35 is held centered over the caisson 20 by anchor cables 36 extending to remote anchors (not shown), with winches 37 adjusting the tension in the respective anchor cables to maintain the vessel 35 correctly positioned. The blowout preventer and drilling head 45 is attached to the top of surface casing 23 and remotely controlled by hydraulic lines 46 extending upward to vessel 35. For circulation of drilling fluid, a riser pipe 47 extends between drilling head 45 and the deck of the vessel 35.

Upon completion of drilling, the head 45 is merely exchanged for the control head 33 with any additional strings of casing being centralized within and suspended from the slip bowl 29 concentrically within the surface casing 23.

A particular benefit of the invention, however, is that if an emergency arises during the progress of drilling that requires quick removal of vessel 35 from the location, it is not necessary to pull the drill pipe 48 out of the hole. It may simply be suspended by slips in drilling head 45 with the blowout preventers closed around it, provided it is centralized in casing 23 by centralizers 25 or by small clearance, without danger of causing excessive bending of caisson 20 due to the drill pipe weight. Then, upon disconnecting control lines 46 and riser pipe 47 and stowing them aboard the vessel, it is ready for removal except for casting off anchor lines 36 or other appropriate measures.

FIGURE 12 shows how the principles of the invention can advantageously be applied in a production platform. In the development of offshore leases producing oil and gas, it is conventional to drill directionally a number of field development wells from a single platform or location. The production from several wells is frequently treated and stored on a platform supported at a safe height above the water surface.

An embodiment of such a platform taking advantage of the principles on which the present invention is based comprises the platform 41 supported by a plurality of columns 20 of tubular form driven into the marine bottom 21 and extending to a safe height above the surface of water 32. The platform 41 carries all of the necessary producing equipment, such as storage 42, separator vessels 43, and the like. Instead of drilling the field wells through separate casings, each is drilled through one of the supporting-column caissons 20. Upon then hanging each casing string within the corresponding column 20 and providing each caisson or casing string with small clearance or a sufficient number of centralizers 25 to maintain the casing 23 centered therein, the platform 41 can then safely support both the producing equipment and the large weight of the well casings. Although the vertical member ab of each caisson 20 between the successive points a and b of attachment of the structural cross bracing 44 (i.e., the

joints of the structure) is subjected to very high compression due to the casing weight, this compression does not contribute to bending of the member ab. By thus being able to neglect bending stresses due to the weight of the casing strings in the design of the platform and its supporting caissons, substantial savings in weight, bulk, and expense of construction of the platform can be made.

In FIGURE 13 is shown a preferred form of the invention which minimizes both weight and expense by most eificient utilization of the structural materials. In this embodiment, the caisson structure comprises two concentric tubular members, an outer member 20a and an inner member 20b. Preferably the inside diameter of member 20b is just slightly larger than the well easing subsequently to be installed therein, so that a minimum but adequate clearance exists between the outside of a well casing and the inside of inner member 2011. Unlike the constant diameter of inner member 20b, the diameter of outer member 20a varies from top to bottom, being narrowest at the top where it just encircles and is attached to the inner member 205, and widest at the level of mud line 21, below which it may have a constant diameter to whatever depth is required to find stable support. Spaced along the length of the column in the annular space between members 20a and 20b are a plurality of braces 50 which rigidly interconnect members 20a and 2012.

For use as an example only, FIGURE 14 shows computed optimum design curves for a column of the type shown in FIGURE 13 wherein allowances have been made for a certain external or static load and for maximum storm-wave forces, and the combined stresses of compression and bending are everywhere uniform throughout the structure. That is, the sum of the compression and bending stresses is a constant, the effect of the casing weight being included in the compressive, but not the bending stress in accordance with the invention.

As is suggested by its correlated position with FIGURE 13, FIGURE 14 refers to a column of total length 550 feet, of which 150 feet are below mud line 21, installed in water 500 feet deep, so that the top of the column is 100 feet below the water surface. C-urve B represents the diameter of inner member 20b chosen to fit with small clearance around the outside of a well casing later to be suspended in the structure, and is shown as of constant diameter throughout its entire length. Curve A shows the necessary variation in diameter of the outer member 20a to provide the uniform stress condition, with allowance for storm waves and a certain static load. Thus, the outer member 20a is of the same diameter as inner member 201) at the top of the structure and increases in diameter more or less uniformly to a value of about 96 inches at a 400-foot depth below the column top, the column thereafter retaining the same diameter for the 150-foot length below mud line 21.

Curve A shows how the diameter of outer member 20a would have to be increased if a load of 460,000 pounds, representing the weight of the typical well casing string or strings were to be stably supported by the column structure at its top. This requires that column 20a increases in diameter quite rapidly near its top and then somewhat more slowly in a manner generally parallel to curve A, until a final diameter of somewhat more than 140 inches is reached. If however, in accordance with the invention the 460,000 pounds of casing are attached at bowl 29 and so supported as to have small clearance inside inner member 20b, then the diameter of outer member 20a need be only that shown by curve A, as the entire weight of the casing strings increases only the compressional stresses in the structure and adds substantially nothing to the bending stresses. That is, the difference in diameter values at any depth between curves A and A represents the saving in weight, size, and expense of the outer structural member 20a due to the utilization of the present invention.

This is shown even more clearly in FIGURE 15. In this figure, curve C shows the maximum diameter of outer member 20a for various depths of the top end of the caisson 20 below the water surface, where the total water depth is 500 feet as was assumed in FIGURES 13 and 14. In other words, this curve shows that if the well head is at somewhat greater depths than 100 feet, i.e. if the caisson height is somewhat less than 400' feet, the maximum diameter requirement of outer caisson member 20a at the mud line decreases markedly. For example, it is only about 54 inches instead of 96, if the top of caisson 20 is placed 200 feet below the water surface.

Curve C shows the variation in maximum diameter of outer caisson member 20a if it is assumed that the weight of the casing strings is applied to the top end of the column rather than as tension in a casing with small clearance or centralization. As is apparent, this maximum diameter decreases also as the top of the caisson is placed at greater depths below the water surface with a resulting decreased height of the caisson. The space between these two curves represents the difference in maximum diameter and corresponding saving made by use of the present invention. Consideration of actual magnitudes of diameter involved shows that the diameter reduction is from about 33 to over 50 percent, based on the values of curve C.

A similar difference is shown by Curves D and D of FIGURE 16, which respectively show the total weight of the caisson in pounds, assuming penetration of the maximum-diameter portion to a depth of 150 feet below mud line 21. Due to the weight of this portion of the caisson structure, the difference between curves D and D is not so great on a percentage basis as between curves C and C, the saving in weight ranging from about 32 to about 41 percent. Obviously, however, due to the greater ease of manufacturing and handling the smaller and lighter structure, as compared with the more bulky and heavier structure required in the absence of this invention, the real saving in cost of the structure is a larger percentage than either the saving in diameter or weight.

While the water depth of 500 feet and the position of the caisson top at feet of depth below the water surface have been thus shown and described as a typical embodiment of the invention, it will be undersood that its advantages are not limited to a narrow depth range of this magnitude. In general, utilization of the invention in the design of marine supporting structures affords some saving in substantially all cases, though for shallower water depth the advantage may be less pronounced, while for greater water depths it is even more outstanding than in the example given.

As an alternative and also preferred embodiment of my invention, the members 50 of FIGURE 13 act not only as rigid internal bracing within the annulus between members 20a and 201) but also form bulkheads dividing this space into waterand air-tight compartments, by which the buoyancy of the structure may be adjusted or varied at the respective locations along its length. Then, by providing appropriate valves and piping in a manner well known in the art, water may be admitted into or exhausted from any of the various compartments to aid in handling and transportation of the caisson structure. For example, the buoyancy of air-filled compartments 51 may either in part or entirely offset the weight of the structure, while the flooded compartments 52 insure that the structure will remain in place below mud line 21 and not rise to the water surface due to excess buoyancy.

By thus offsetting the weight of the caisson through buoyancy the tendency of the column to bend and buckle under its own weight is eliminated. Then, as the bending effect due to the weight of the casings is substantially cancelled by proper centralization or use of small clearances in accordance with the present invention, the only bending tendency comes from the weight of the wellhead equipment. This, however, is usually not very large compared to the weight of the caisson itself and of the well casings, so that by a proper distribution and amount of buoyancy, the caisson structure of FIGURE 13 can be adapted for use in almost any depth of water.

While my invention has been described by reference to the foregoing specific details and examples, its scope should not be considered as limited thereto, but is properly to be ascertained from the scope of the appended claims.

I claim:

1. A stable load-supporting marine structure comprising an elongated vertical hollow compression member having its bottom end fixed solidly in the earth below the marine bottom and its upper portion extending for a substantial distance through the water to a point in the vicinity of the water surface, the strength and stability of said hollow member being sufficient to support substantially only its own weight and any external loads or forces imposed on it, and a tension member for carrying additional weight extending below said bottom end of said hollow member, the combined loads carried by said hollow and said tension members substantially exceeding that which said hollow member alone may support as an externalload without buckling, said tension member being attached to the center of the upper end of said hollow member and having a small clearance within said hollow member at at least one point intermediate its ends, so that the tendency of said hollow member to bend due to said additional weight brings said members into contact at least at said one point to substantially prevent increased bending of said hollow member by said additional Weight.

2. A stable load-supporting marine structure as in claim 1 in which said tension member has a small clearance with said hollow member at a plurality of spaced points intermediate the ends of said hollow member, so that the tendency of said hollow member to bend due to said additional weight brings said members into contact at at least some of said points to substantially prevent increased bending of said hollow member by said additional weight.

3. A stable load-supporting marine structure as in claim 1 in which said tension member has a small clearance with said hollow member substantially throughout the length of said hollow member, so that the tendency of said hollow member to bend due to said additional weight brings said members into contact over a substantial portion of the length of said hollow member to substantially prevent increased bending of said hollow member by said additional weight.

4. A stable load-supporting marine structure as in claim 1 comprising also centralizing means between said members at least at said one point for providing said small clearance at said point.

5. A stable load-supporting marine structure as in claim 2 comprising also a plurality of vertically spaced centralizing means between said members at said plurality of spaced points for providing said small clearance at said points.

6. A combination column for marine wells comprising an outer vertical hollow compression member stable under its own weight and any additional external loads and forces to which it may be subjected, said member extending for a substantial distance through the water above a marine bottom, and an inner tension member attached to said outer member at the center of its upper end, said tension member extending below the lower end of said outer member and supporting, as a substantial additional load different from and in addition to said weight and external loads and forces, the well equipment below said marine bottom, the clearance between said outer and inner members being small enough so that a small amount of bending of said outer member under said additional load brings said members into contact at at least one point substantially below the top of said outer member to substantially prevent further bending of said outer member by said additional load.

7. A marine well structure comprising a well casing, a caisson in compression surrounding said casing and extending from a load-supporting depth below the marine bottom upward for a substantial distance above said bottom to a point in the vicinity of the water surface, auxiliary stabilizing means acting on said caisson to prevent it from buckling under the weight of itself and of external loads and forces which may act on it, means suspending said casing from the center of the top of said caisson, and at least one point of small clearance between said casing and said caisson in the vicinity of the marine floor to substantially prevent bending of said caisson by the additional load represented by the tension in. Said casing.

8. A marine well structure as in claim 7 in which said auxiliary stabilizing means comprise a plurality of equiangularly spaced guy cables extending from said caisson at a depth substantially above the marine floor to anchor points horizontally spaced from said caisson on the marine floor.

9. A marine well structure as in claim 7 in which said auxiliary stabilizing means comprise at least one buoyancy chamber attached to said caisson in the vicinity of its upper end and buoyantly supporting a substantial part of the caisson weight and external load.

10. A stable load-supporting structure for marine wells comprising a well casing, a caisson surrounding said casing and extending upwardly from a load-supporting depth below the marine floor, said casing being supported from the center of the upper end of said caisson, said caisson having two concentric hollow members, means in the annular space between said members for maintaining them concentric, the inner of said members having a constant internal diameter slightly larger than the outer diameter of said well casing suspended therein, the outer of said members having a diameter increasing generally with depth below the water surface such that the sum of the compressive and bending stresses in the caisson due to the caisson weight and to the maximum known and anticipated external loads and forces, plus the compressive but not the bending stress imparted to the caisson by the casing weight, is substantially constant throughout said caisson, the small clearance between said casing and said inner hollow member being such that a small amount of bending of said caisson due to the casing weight brings said casing and said inner member into contact throughout a substantial portion of the length of said inner member to substantially prevent increased bending of said caisson by the weight of said casing.

11. A structure as in claim 10 wherein said maintaining means comprise spaced fluid-tight bulkheads subdividing the annular space between said inner and outer members into buoyancy compartments.

12. A structure as in claim 11 wherein said caisson extends from a load-supporting depth below the marine floor to within diving range from the water surface.

13. A structure as in claim 11 including also auxiliary stabilizing means acting on said caisson to prevent it from buckling under the weight of itself and of said external loads and forces which may act on it.

14. A structure as in claim 11 wherein said well casing is substantially smaller in diameter than the internal diameter of said inner member, and including also a plurality of vertically spaced centralizing means between said casing and said inner member to substantially prevent increased bending of said caisson by the weight of said casing.

References Cited by the Examiner UNITED STATES PATENTS 157,908 12/ 1874 Cornell 138-115 2,236,682 4/1941 Gross 6146.5 2,512,783 6/ 1950 Tucker --6 2,598,329 6/1952 Wilson 6146.5 3,017,934 1/1962 Rhodes et al 1757 X 3,166,125 1/1965 Hubby 16675 X ERNEST R. PURSER, Primary Examiner. 

1. A STABLE LOAD-SUPPORTING MARINE STRUCTURE COMPRISING AN ELONGATED VERTICAL HOLLOW COMPRESSION MEMBER HAVING ITS BOTTOM END FIXED SOLIDLY IN THE EARTH BELOW THE MARINE BOTTOM AND ITS UPPER PORTION EXTENDING FOR A SUBSTANTIAL DISTANCE THROUGH THE WATER TO A POINT IN THE VICINITY OF THE WATER SURFACE, THE STRENGTH AND STABILITY OF SAID HOLLOW MEMBER BEING SUFFICIENT TO SUPPORT SUBSTANTIALLY ONLY ITS OWN WEIGHT AND ANY EXTERNAL LOADS OR FORCES IMPOSED ON IT, AND A TENSION MEMBER FOR CARRYING ADDITIONAL WEIGHT EXTENDING BELOW SAID BOTTOM END OF SAID HOLLOW MEMBER, THE COMBINED LOADS CARRIED BY SAID HOLLOW AND SAID TENSION MEMBERS SUBSTANTIALLY EXCEEDING THAT WHICH SAID HOLLOW MEMBER ALONE MAY SUPPORT AS AN EXTERNAL LOAD WITHOUT BUCKLING, SAID TENSION MEMBER BEING ATTACHED TO THE CENTER OF THE UPPER END OF SAID HOLLOW MEMBER AND HAVING A SMALL CLEARANCE WITHIN SAID HOLLOW MEMBER AT AT LEAST ONE POINT INTERMEDIATE ITS ENDS, SO THAT THE TENDENCY OF SAID HOLLOW MEMBER TO BEND DUE TO SAID ADDITIONAL WEIGHT BRINGS SAID MEMBERS INTO CONTACT AT LEAST 